We parametrize the energy-momentum vector of a massive particle of mass \(m\)  as [1]}, [2]},
\(p^{\mu } (y,\vec{x}) = m \bigg ( \frac{1+y^2 +\vec{x}^2}{2y} , \frac{\vec{x}}{y} , \frac{1 - y^2 - \vec{x}^2}{2y} \bigg )\) 
